Emulsion are a mixture of two or more immiscible liquids: One liquid forms a continuous phase in which the other liquid is dispersed in the form of small drops. Commonly, the continuous phase is water or another polar solvent and the dispersed phase is an oil. However, other possibilities exist including inverted emulsions (water dispersed in oil) and double emulsions (water in oil in water). Water-in-oil-in-water (W/O/W) double emulsions are vesicular systems composed of small aqueous droplets contained in larger oily droplets which, in turn, are dispersed in a continuous aqueous phase. The applications of these systems range from pharmaceutical drug delivery to foods and cosmetics. Some benefits of these emulsions include the protection of the entrapped substances, their capacity to incorporate several actives in the different emulsion compartments, and their sustained release effect.
Two important release mechanisms are often cited in the literature: i) the diffusion of the active agent through the oily barrier; and ii) the droplets bursting that occurs either by dilution in a hypo-osmotic solution (with regards to the internal aqueous phase) or by submission to a shear stress.
Deformation under shear and the bursting of simple, dilute emulsions has been investigated. The break up occurs when shear stress exceeds cohesion stress, which is defined as a capillary number Ca=ηcGr/σ, where ηc is the continuous phase viscosity, G the shear rate, r the radius of the globule at rest and σ the interfacial tension between oil and water. Bursting occurs when this capillary number exceeds a critical value Cacr close to unity. This relation points out that the thickening of the continuous phase is an important factor to encourage the globules bursting.
Thus, thickening of the external aqueous phase of water-in-oil-in-water (W/O/W) multiple emulsions is desired to increase the release under shear during use. However, it leads to the micelles bursting during fabrication because the additional viscosity causes the emulsified droplets to rupture under ordinary fabrication conditions.